It is possible to detect free charge carriers in solids using scanning capacitance microscopy technology (SCM). Free charge carriers are found e.g. in metals. But doped silicon, as is used in the entire semiconductor industry, also contains free charge carriers (of a type different than in metals). It is thus also possible to use this property to analyze electronic components, which e.g. include dielectric layers.
With the increasing miniaturization of structures in integrated circuits, the tolerances for the fabrication of doped areas are becoming smaller and smaller. In addition to the spreading-resistance method, SCM measurement of structures represents a simple possibility for quantitatively investigating doped zones.
In this process, an electrically conductive measuring tip scans the surface of the component or substrate and registers the capacitance between the tip and the sample as a function of the location, so that capacitance changes in the underlying substrate of the component can be detected. A silica layer serves to isolate the substrate, and simultaneously functions as a dielectric.
The functionality of such SCM technology is only assured when the tip of the probe is in constant contact with the material sample to be scanned. The forces acting laterally on the tip produce noticeable tip wear of the probe during the scanning process. Additionally, the constant contact of the tip with the sample also exerts forces on the sample that can damage it. For the same reason, it is not possible to test elastic samples with this method.
A scanning method has previously been described in K. Goto, K. Hare, Tapping Mode scanning capacitance microscopy, SPIE Vol. 3009, 84 (1997) and in K. Goto, K. Hare Tapping Mode scanning capacitance microscopy, Rev. Sci. Instrum. 68 (1), January 1997 in which the probe oscillates above the surface of the sample and touches it only briefly (tapping mode). In this tapping mode, the tip of the probe is periodically brought into contact with the substrate so as to exert a force on the atomic scale (thus also the name IC-AFM=intermittent contact atomic force microscopy) and excited with a frequency which is close to the resonance frequency of the probe, as has also been described e.g. in R. Garcia, Dynamic Atomic Force Microscopy Methods, Surface Science Reports 47 (2002), 197-301.
Interactions between the surface of the sample and the tip cause the amplitude of the tip motion to change. Using a control loop a signal is generated which varies the distance between the tip and the sample such that the amplitude of the tip motion remains constant. This signal is a measure of the topography of the sample surface. As a consequence, the tip periodically touches the surface and only briefly each time. As the lateral scanning speed is much slower than the vertical tip speed, the lateral forces between the tip and the sample are practically negligible.
The structure to be measured essentially includes a system of two capacitances connected in series. A part of the total capacitance is formed by the substrate capacitance and the overlying silica. The remaining portion of the capacitance consists of the tip and the air between the tip and the substrate as a dielectric. Whereas the capacitance of the substrate together with that of the silica represents a portion that varies only with the sample properties (such as the doping), the value of the capacitance formed by the tip and the air is approximately reciprocal to the distance between the tip and the surface of the silica. As the conductive tip in turn is connected to a capacitance sensor, an amplitude-modulated signal is generated at the sensor output whose frequency is equal to the resonance frequency of the tip and whose degree of modulation changes with the substrate capacitance.
The advantage of this method is that lateral forces on the probe tip are substantially reduced and the probe tip wear is thus minimized. The mechanical advantage arising from the dynamic embodiment, however, is offset by significant disadvantages with respect to the SCM signal. For one thing, non-local couplings e.g. between the tip holder and the sample also contribute to the modulated signal, and for another the dynamics of the measurement signal are weakened by crosstalk on the oscillator signal driving the probe. The resolution of dynamic SCM thus remains much lower than the topographic resolution of the component or the substrate.
A further development has been described in the monograph “Intermittent contact scanning capacitance microscopy—An improved method for 2D doping profiling” by P. Breitschopf, G. Benstetter, B. Knoll, W. Frammelsberger, published in the periodical Microelectronics Reliability 45 (2005), 1568-1571. This method is based on the fact that, due to the harmonic mechanical tip motion and the non-linear distance-dependency of the SCM signal, higher-order spectral components are generated. The higher-order signal relevant for the measurement is detected by means of a two-phase lock-in amplifier. The tapping signal represents the reference frequency, which, before being used with (or in) the lock-in amplifier, must be multiplied by a factor of between 2 and 4 or higher to minimize primarily harmonic signal components of non-local coupling e.g. between the tip holder and sample, on the one hand, and the background interference due to feedover of the tapping signal on the other. This complex has been described in G. Wurtz, R. Bachelot, P. Royer, Imaging a GaAlAs laser diode in operation using apertureless scanning near-field optical microscopy, The European Physical Journal—Applied Physics 5 (1999) 849-854 and in B. Knoll, F. Keilmann, Enhanced dielectric contrast in scattering-type scanning near-field optical microscopy, Optics Communications 182 (2000) 321-328.
All previously known variants of scanning capacitance microscopy use a UHF (ultra high frequency) capacitance sensor for the actual capacitance measurement. This includes a UHF resonant circuit and must be tuned to match the respective ambient conditions before each measurement. However, in this process it is not possible to rule out an interaction with the environment, particularly under variable ambient conditions. Additionally, scanning probe microscopes are not normally equipped with UHF capacitor sensors. These must be retrofitted in the form of expensive capacitance modules and adapted to the existing evaluation electronics.